1. Field of the Invention.
The present invention relates generally to electromagnetic wave reflectors having a three-dimensionally curved reflecting surface, and particularly relates to a structure of this type where the reflecting surface is formed by a pair of open mesh conductors.
2. Description of the Prior Art.
Electromagnetic wave reflectors such as for example parabolic radar antennas require electrically conducting surfaces which closely conform to mathematically calculated shapes. Deviation of the reflecting surface from the calculated three-dimensional surface causes loss of power and loss of information. Desirably the reflecting surface forms an optical finish which may be defined as a surface which deviates no more than 5 microinches RMS (root mean square) from the calculated surface.
Conventionally such reflecting surfaces for antennas, reflectors and the like are manufactured by machining, spinning, stretch forming or vacuum plating of metals on plastic members. Generally such antennas are expensive and usually require much tooling and labor. In addition reflectors which are machine spun or stretch-formed may be too heavy for use in aircraft or other applications requiring lightweight reflectors. This is also true of radar antennas such as search antennas which must be mechanically moved. Another problem is that the reflecting surface is exposed and subject to physical damage. When they are used on ships they are subjected to corrosion by sea water spray and the like.
To overcome some of these problems electromagnetic wave reflectors have been proposed which employ thin woven wire or metal strip surfaces. The metal may be laminated within a lightweight reinforcing plastic support. It is well known that if the mesh size is appreciatively less than the length of the electromagnetic waves to be reflected, the porous surface reflects as if it were continuous. Examples of such reflector structures are disclosed in the patents to Wild et al U.S. Pat. No. 2,945,233, Mondano U.S. Pat. No. 3,150,030 and Hart, Jr. U.S. Pat. No. 2,948,896. According to the Hart, Jr. patent however, the conductive wire screen is covered on both surfaces with fiberglass cloth layers. Therefore the conductive screen is relatively far away from the outer surface and its shape cannot be controlled as precisely as may be necessary.
According to these patents the woven wire metal strip and the fiberglass reinforced plastic resins are formed over molds having the predetermined convex surfaces. Sometimes vacuum techniques are employed to force laminations against the surface of the mold. Comparatively lightweight electromagnetic wave reflectors having protective reflecting surfaces may thereby be formed. However, the exact shape of the reflecting surface is difficult to control in this manner.
A major problem associated with the use of woven wire or metal strips for the reflecting surface is that the woven materials cannot be made to smoothly conform to a three-dimensionally curved surface. Woven materials stretch only diagonally and tend to wrinkle, bulge or fold back on themselves when stretched over three-dimensionally curved surfaces. The resulting surface irregulatiries cause power loss and may result in lost information. This tendency to cause surface irregularities is enhanced by the laminations of the reflecting material between layers of still soft reinforcing resins. Further it will be realized that vacuum forming equipment increases the price of such antennas.
Reference is also made to a patent to Spong U.S. Pat. No. 1,962,041. This patent proposes to utilize a tubular length of mesh to be used as an antenna or earth. The antenna actually consists of suitable wire of copper or phosphor-bronze which is knit in the form of a stocking to provide a tubular structure. However, the patent does not disclose any manner in which such an antenna may be protected from environmental influences and contaminations nor does the patent disclose that such an antenna may be formed into a three-dimensional surface of predetermined contour.
The patent to Hubbard U.S. Pat. No. 3,536,800 is also noted. According to this patent a thin metallic layer of film is laid upon a mold. A thick plastic support layer is laid over the metal film. The metal film is made by spraying fine particles of metal onto the mold surface.